In the past, considerable difficulty has been experienced in connection with the failure of rocket motors using graphite or coated graphite rocket nozzle inserts. The smaller diameter rocket motors where the inside diameter of the insert is of the order of 1" have not been especially troublesome, but larger diameter inserts in the 7" to 12" or higher diameter range have experienced frequent failure in fracture, and it is believed that the reasons for this type of failure can be traced to a lack of understanding of the complex problems resulting from expansion of the inserts during firing of the rocket motor attributable both to temperature and pressure causes. The stresses induced in the nozzle insert include thermal stresses, pressure stresses, and stresses caused by restraint imposed by the press of the backside surface against the insert as the insert expands during heating. The thermal stresses which are induced are caused by the thermal gradient through the thickness of the insert and are compressive on the hot inside surface of the insert, but are tensile on the outer surface of the insert. Whether the insert is graphite, or graphite coated with pyrolytic graphite, the thermal stress in a typical motor insert (based on elastic properties) can exceed the allowable strength of the graphite under the severe thermal conditions resulting during firing of the motor. These thermal stresses may not by themselves exceed the allowable stress, but they at least approach it. Such thermal stresses are additive to the pressure stresses which are also induced in the insert during the firing of the rocket motor as a result of increase in pressure on the inside surface which produces additional tensile stresses on the outer surface of the insert. Such tensile stresses, attributable both to thermal and pressure conditions, can easily exceed the total stresses which the graphite can withstand.
It is recognized that for graphite inserts acceptable stress levels may possibly be achieved by backing up the annular graphite ring with a supporting member. If this supporting member has sufficient compressive strength, then the tensile stress on the outer periphery of the nozzle insert can be reduced by the support of the back-up member, but it is also true that where a stiff back-up member is used to support the nozzle insert, it may not permit the insert to grow sufficiently, and this may cause other destructive stresses within the nozzle insert. In other words, if the back-up material does not compress, then the restraint on the nozzle insert preventing it from expanding will add to the compressive stresses on the inside of the rocket nozzle insert which can also fail, for instance by "chunking".
The difficulties involved in providing an optimum back-up material, even after the problems have been recognized, are increased by the fact that the material which is provided is itself fully confined against escape in any direction, and therefore, it must have a compressive capability of its own. Moreover, there is the additional problem attributable to the very high temperature under which the back-up material is required to perform, such temperatures normally reaching and exceeding 3,000.degree. F.